Place the popcorn into a large metal mixing bowl that has been sprayed with cooking spray. Remove any unpopped kernels. Set aside.
In a heavy bottomed saucepan over medium-high heat, melt together the butter, brown sugar, corn syrup and salt. Bring to a boil, stirring constantly, then lower the heat and allow to bubble for 5 minutes.
After 5 minutes remove the saucepan from the heat and add 1 teaspoon of baking soda. The baking soda will cause the caramel to foam. Stir constantly until the foam begins to die down. Add 1 teaspoon of pure vanilla.
Pour the caramel over the popcorn. Using a large spoon, stir the popcorn until it's evenly coated with the caramel. Continue to stir until the mixture has cooled enough to handle. This step will take around 2 minutes.
Wearing gloves lightly spritzed with cooking spray, divide the mixture evenly forming into 3 inch balls. Press together firmly and place onto wax paper to set-up. Allow to sit until completely cooled and set, around 20 minutes.
Melt the milk chocolate chips and heavy cream together in the microwave in 20 second increments, stopping to stir each time. Repeat this process until the chocolate is completely smooth.
Dip each popcorn ball into the melted chocolate covering the top. Sprinkle with nonpareils while wet. Allow to sit for 45 minutes or until the chocolate is set.